1. Field of the Invention
The present invention relates to a muting control circuit and, more particularly, to the improvement in muting characteristics.
2. Description of the Prior Art
In audio systems or the like, a muting circuit is provided for eliminating click noise which is generated at the moment of throwing switch of the system. In FIG. 1 there is shown a conventional muting control circuit. A terminal 2 is an input terminal for inputting a signal to be muted, and a terminal 4 is an output terminal. In Response to a high current I.sub.o outputted from a control circuit 6, the terminal 4 outputs less, thus muting is performed.
The control circuit 6 is provided for varying the output current I.sub.o depending upon the voltage at a controlling input terminal T.sub.c. The control circuit 6 allows the output current I.sub.o to be increased with increasing voltage of the controlling input terminal T.sub.c to the maximum point, as shown in FIG. 2. It also allows the output current I.sub.o to be decreased in the case of a further rise in voltage. Muting is performed by the output current I.sub.o.
An external resistance R.sub.1 and an external capacitor C.sub.1 are connected to the controlling input terminal T.sub.c. A curve 10 in FIG. 3A shows the voltage at the controlling input terminal T.sub.c in the case where voltage is applied to one end of the resistance R1 or a terminal 8. By applying the voltage, charging of the external capacitor C.sub.1 is initiated. The voltage V.sub.p at the controlling input terminal T.sub.c increases with increasing time according to a curve 10 in FIG. 3A. At the moment when the voltage V.sub.p exceeds a first threshold V.sub.TH1, the output current I.sub.o is allowed to flow, so that muting is initiated (.alpha. in FIG. 3B). The output current I.sub.o increases with increasing voltage V.sub.p, and after a peak .beta., it decreases. Further, when the voltage V.sub.p exceeds a second threshold V.sub.TH2, the output current I.sub.o becomes equal to zero, thus muting is achieved (.gamma. in FIG. 3B).
As mentioned above, by varying the terminal 8 voltage from an L level to an H level, muting can be performed for a desired time.
Muting also occurs when, the terminal 8 voltage at the H level is lowered to the L level. This is carried out by varying the voltage V.sub.p according to a curve 12 shown in FIG. 3A. As a result, the output current I.sub.o varies as shown in a curve 14 of FIG. 3B. However, the conventional muting circuit described above has the following problem.
In order to raise the terminal 8 of L level to H level (usually source voltage), a voltage of H level is applied to the external capacitor .sub.1. This causes the external capacitor C.sub.1 to be charged rapidly, so that the voltage V.sub. at the controlling input terminal T.sub.c is made to increase rapidly (curve 10 in FIG. 3A).
On the contrary, when the terminal 8 of H level is lowered to L level, the external capacitor C.sub.1 at the level of a charged voltage discharges. The external capacitor C.sub.1, however, is such that it is not charged by the applied voltage to the terminal 8, but by a divided voltage which is determined by dividing the applied voltage using R.sub.1 and R.sub.2. As a result, discharge of the external capacitor C.sub.1 is carried out more slowly as compared with the case of charge thereof.
This means that there is the difference in muting time between the case where the terminal 8 of L level is raised to H level (when charging the capacitor C.sub.1.) and the case where the terminal 8 of H level is lowered to L level (when discharging the capacitor C.sub.1). Consequently, if the muting is the best suited for the change from H level to L level, the output current I.sub.o is made to change too sharply from L level to H level, as shown in FIG. 3B. For this reason, there is the possibility of shock noise.